The speed, motion, and direction of seismic waves changes dramatically at different levels within Earth, known as seismic transition zones. Therefore, scientists can make various assumptions about the earth's character above and below these transition zones through careful analysis of seismic data. This information reveals that Earth is composed of three basic sections, the crust (the thin outer layer), the mantle, and the core.
The outermost layer of Earth is the crust, or the thin "shell" of rock that covers the globe. There are two types of crust: the continental crust, which consists mostly of light-colored rock of granitic composition that underlies the earth's continents; and the oceanic crust, which is a dark-colored rock of basaltic composition that underlies the oceans. One of the most important differences between continental and oceanic crust is their difference in density. The lighter-colored continental crust is also lighter in weight, with an average density of 2.6 g/cm3 (grams per cubic centimeter), compared to the darker and heavier basaltic oceanic crust, which has an average density of 3.0 g/cm3. It is this difference in density that causes the continents to have an average elevation of about 2,000 ft (600 m) above sea level, while the average elevation (depth) of the ocean bottom is 10,000 ft (3,000 m) below sea level.
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